1. Field of the Invention
The present invention relates to a solid-state image sensing device which is capable of imaging all pixels at the same timing. Particularly, the invention relatives to the solid-state image sensing device having an electronic shutter function.
2. Description of the Related Art
Solid-state image sensing devices which are conventionally used are roughly classified into CCD type and MOS type according to units which read photocharges generated at a photoelectric conversion element. CCD type devices accumulate photocharges in a potential well and simultaneously transmit them. MOS type devices read electric charges accumulated in a pn joint capacity of a photodiode through an MOS transistor.
In the MOS type solid-state image sensing devices, after each pixel is imaged by vertical scanning and horizontal scanning, image signals output from the pixels, respectively, are output serially from the solid-state image sensing devices. The timing, therefore, at which each pixel is imaged differs from each other. When a subject which changes momently is imaged, the imaging cannot be carried out on each pixel under the same condition, and in result image distortion occasionally occurs.
As prior solid-state image sensing devices, therefore, a device, in which two integrating circuits are provided and the imaging operation is performed on all the pixels at the same timing, is suggested (see U.S. Publication of Patent Application No. 2002/0054389). A structure of the pixels in the prior solid-state image sensing device is shown in FIG. 22. The pixel in FIG. 22 has a photoelectric conversion circuit 100, a capacitor C1, a capacitor C2, an MOS transistor T4, an MOS transistor T5, an MOS transistor T6, and an MOS transistor T3. The photoelectric conversion circuit 100 generates an electric signal according to incident light, and the capacitor C1 integrates the electric signal. The capacitor C2 samples and holds the electric signal integrated by the capacitor C1, and the MOS transistor T4 amplifies a current of the electric signal sampled and held by the capacitor C2. The MOS transistor T5 electrically connects and disconnects the capacitors C1 and C2, and the MOS transistor T6 serves as a switch for resetting the capacitor C2. The MOS transistor T3 serves as a switch for outputting the electric signal from the MOS transistor T4 as an image signal.
In the solid-state image sensing device having the pixels of the structure in FIG. 22, the photoelectric conversion circuits 100 and the MOS transistors T5 on all the pixels operate at the same timing, and after the electric signals obtained by performing the imaging operation at the same time are integrated by the capacitors C1, they are sampled and held by the capacitors C2. After the MOS transistors T5 are turned off, the pixels are scanned in the horizontal and vertical directions, and image signals according to the electric signals sampled and held by the capacitors C2 are amplified and output from the pixels.
In the solid-state image sensing device having the structure in FIG. 22, a DC voltage VRS which does not change is always applied to a source of the MOS transistor T6. For this reason, even if the MOS transistor T6 is OFF, a leak current flows in a voltage supply line for supplying the DC voltage VRS from the capacitor C2 via the source-drain of the MOS transistor T6. The leak current from the capacitor C2 increases as ambient temperature of the solid-state image sensing device becomes higher. The leak current is determined by a potential difference between the source and the drain of the MOS transistor T6 and ON resistance of the MOS transistor T6.
Even if, therefore, the electronic shutter function, which resets the photoelectric conversion circuits 100 and turns the MOS transistors T5 ON on all the pixels of the solid-state image sensing device at the same timing, is used, after the electric signals are sampled and held by the capacitors C2, they are read sequentially on the respective pixels. A quantity of the leak current from the capacitors C2 on the pixels differ according to reading time. Since the leak current from the capacitors C2 on the pixels differs, in result when image data obtained from the solid-state image sensing device are reproduced, shading noise which becomes image distortion occurs. Since a quantity of the leak current changes due to a change in the ambient temperature, a strength of the image signals to be output is influenced by the ambient temperature.